<k-upload>-onSuccess: 2nd argument should contain api response but does not

The CMS Questions
1

Having my tiny custom field:

<template>
    <div>
      <k-image :src="value" />
      <k-upload ref="upload" @success="onSuccess" />
      <k-button icon="upload" @click="upload">Upload</k-button>
    </div>
  </template>

and some methods with it:

  methods: {
    upload() {
      this.$refs.upload.open({
        url: window.panel.api + '/media/freePicturesPost',
        accept: "image/*",
        multiple: true
      });
    },
    onSuccess (uploads, response) {
      // i.e.
console.log(uploads);
console.log(response);
      // etc.
    }
  },

Uploading my file works fine; here is the response I get from my own API endpoint:

{"foo":"bar","payload":"https:\/\/xxxblob.core.windows.net\/path\/apfel.jpeg"}

What I am wondering is that in onSuccess() the second argument is undefined, but should contain the API response according to
https://getkirby.com/docs/reference/plugins/ui/upload

The first parameter is not empty, but does not contain the api response.

Why that?